Porous body model for predicting temperature distributions in wire wrapped fuel and blanket assemblies of a LMFBR

Abstract

Existing methods of thermal analysis of a wire wrapped rod bundle of a Liquid Metal Fast Breeder Reactor are based on the principle of subchannel analysis. A model for thermal transport in wire wrapped rod bundles is developed here. The model is similar in principle to the one which has long been successfully used in chemical engineering for heat and mass transfer in fixed beds of packed solids. By dividing the bundle into two predominant regions and applying the model of a porous body to a LMFBR assembly a simple procedure for calculating temperature distributions in LMFBR fuel and blanket assemblies has evolved. The results obtained were found to predict available data with as good a precision as do the more complex analyses. Correlations for the two empirical constants were obtained as functions of geometric parameters based on an extensive analysis of existing data. The LMFBR fuel assemblies operate in forced convection (negligible natural convection) under steady state conditions whereas the blanket assemblies may operate in forced or mixed convection (combined forced and free convection). Two different formulations of equations, corresponding to these two convection regimes, were developed using the same basic model. The calculation procedure for assemblies in forced convection (called ENERGY I) is considerably simpler than that (ENERGY II, ENERGY III) in mixed convection, where buoyancy effects become important. Therefore it is desirable to use ENERGY I for forced convection (although ENERGY II, III can also be used in forced convection, the computational times are fifteen fold greater). In order to determine when buoyancy effects become important a new criterion is developed. Given the power, the power skew, the operating and geometric characteristics of the bundle, the critical modified Grashof Number predicts when buoyancy effects become important. (auth)